1. Field
The field of the invention is power and power transmission systems for tracked vehicles, and more particularly those utilizing electric power.
2. State of the Art:
Both-mechanical and hydraulic power transmission systems have been used in tracked vehicles to transfer the power from engines to track drive shafts. In the mechanical systems, differentials have been extensively used. In these systems, vehicle steering has been by overdriving the track drive shaft on the side of the vehicle away from the direction of turn, braking the drive shaft on the side toward the turn, or combination of both. Some differentials have also utilized internal braking devices to slow the rotation of the power output shaft linked with the track drive train on the side toward the turn, while increasing the rotation of the opposite power output shaft. Other differential systems have incorporated two oppositely extending track drive power output axles and two rotating braking axles each extending oppositely outside the differential transverse to the vehicle, along with a brake external to the body of the transmission mounted on each of the braking axles. Such a system is disclosed in U.S. Pat. No. 4,377,094. Brakes, generally of the liquid disc type, have been disclosed for directly braking the wheels of vehicles other than tracked vehicles, for example, in U.S. Pat. Nos. 2,940,549, 2,955,682, 2,971,612 and 2,928,504.
Another common means of powering the vehicle tracks employs an internal combustion engine, preferably diesel, to provide pressurized hydraulic fluid. Each track drive shaft is provided with a hydraulic motor individually supplied with pressurized hydraulic fluid from a common reservoir. Since the separate shaft motors are individually controllable, steering is accomplished by powering one or the other of the shafts to a greater degree than the other. The hydraulic motors are reversible, so that sharp turns are possible by reversing the direction of power on the two track drive shafts. The shaft motors of such vehicles normally remain engaged even if steering levers provided are in neutral position, so that positive braking must be applied to each shaft to keep the vehicle stationary, as long as the engine is running.
Both the hydraulic and mechanical systems exhibit inefficiencies including that of the engine and the power train, the mechanical system from mechanical friction losses and the hydraulic system from fluid friction losses resulting in temperature rise in the fluid which must be wastefully dissipated.
Many electrically propelled vehicle concepts have been studied and prototypes of some have been constructed and tested. Individual electric wheel motor propulsion systems, similar to the hydraulic propulsion system described above have also been known. U.S. Pat. No. 4,347, 907 discloses an all electric a.c. powered farm tractor, with wheel induction motors controlled by cyclo-converters. The same reference makes mention of a Le-Tourneau "LetroStaker", a log stacker with wheels driven by d.c. motors. The well known Le-Tourneau earth-moving machine is also powered by wheel motors. Electrically powered tracked vehicles have also been investigated. U.S. Pat. No. 1,863, 504 discloses an electrically driven track-laying vehicle having a pair of tracks individually powered by electric motors acting through drive chains upon a sprocket shaft. A similar system is disclosed in U..S. Pat. No. 2,605,852, intended for use upon electric shovels or mine shuttle cars. U.S. Pat. No. 1,814,996 discloses an electrically powered, tracked cultivator, however utilizing a single motor for driving of both tracks, and U.S. Pat. No. 1,391,529 discloses a similar single motor electrical powered tracked vehicle. Thus, none of the known prior art discloses practical electrical power for tracked vehicles for use upon snow. Those designs which have proved practical include the earth moving and log stacking applications, and perhaps the agricultural applications, all of which not only do not find weight highly objectionable, but even desirable to provide needed traction. Another study, including construction and test of a model, is found in the Journal of Terra Mechanics, Vol. 28, No. 1 pp. 79-85, 1991. The study investigates the use of multiple electric motors to power a track, to reduce tensile stress thereupon. Lip service is paid to lower weight motors, but a large number of motors are used, again relegating the study as applicable to vehicles such as those discussed above. Considerable analysis and design has been expended upon electrically powered vehicles for roadway use. One such effort is described in SAE Technical Paper Series, of the Engineering Society for Advancing Mobility Land, Sea, Air and Space, 400 Commonwealth Drive, Warrendale, Pa. 150960001, 0001, U.S.A., presented at the International Congress and Exposition, Detroit, Mich. Feb. 26-Mar. 2, 1990. The study investigates the use of individual wheel motors in a battery powered system. Again, achieving minimum vehicle weight in the design is not of overriding concern, because of operation of the vehicle upon a firm paved roadway, and the study fails to address any critical design concerns relative to tracked vehicles for use on snow.
Prior art propulsion systems for tracked vehicles are also objectionable because of environmental pollution, the engines of course giving off exhaust pollutants, particulate and gaseous. Both the differential and hydraulic motor systems are subject to polluting leakage of fluid upon the slopes.
There is therefore a need for an improved power system for the tracks of tracked vehicles which is capable of improved efficiency and reduced environmental pollution.